Many production processes having moving webs require the quality of the web to be inspected. There is a need for 100 percent area inspection of large width webs such as paper webs for defects such as holes, cuts, wrinkles, dirt, etc., at web velocities up to 5,000 feet per minute.
The web scanning systems presently available commercially or known to the inventor fall in the categories of point or slit scanners or multihead fixed field detectors. The point scanners include laser and television camera scanners going transversely of the web. The slit scanners are ones which have a slit disposed transversely of the web without any scanning of a beam and depend upon the moving web to establish inspection of the area of the web. The multihead fixed field detectors are ones wherein the individual heads scan only a small area of the web and the cumulative areas from all of the multiple heads lie across the web to permit inspection as the web is moved.
The disadvantages of the prior art scanning devices is that point scanners cannot provide 100 percent inspection of a moving web unless the web is moving at a very slow speed relative to the transversely moving point. The slit scanners, such as image dissectors, can provide 100 percent inspection, but require prohibitively intense and uniform illumination of the total area being scanned. Operation in the visible light spectrum requires special direct current illumination sources to eliminate modulation interference produced by normal alternating current industrial lighting systems. Multihead fixed field detectors are complicated by the fact that they require hundreds of detector/amplifier combinations each of which requires separate sensitivity or balance adjustments. The electrical output for a given web defect is directly proportional to web velocity necessitating a wide-band signal amplification system.
Most of these prior systems require illumination in close proximity to the web, either above or below the web. In many systems the web must be threaded through the sensor. All commercial systems of the prior art are expensive due to the complexity of manufacture, the requirement for special intense through-the-web illumination, or complicated on-line adjustments.
The problem to be solved is to construct a web scanning apparatus which obviates the above-mentioned disadvantages.